Effect Of Two-Step Austempering Process On Phase Transformation Rate,Microstructure And Properties Of Nanobainite Bearing Steel

Nano-bainite bearing steels have gained extensive attention for their excellent properties,however,it takes a long time to obtain all bainite.A 100Cr2Si1NiMo nano-bainite bearing steel is studied,and the bainite transformation is accelerated by different two-step austempering processes.The phase transition kinetics test,microstructure characterization and mechanical properties test are used to reveal the phase change of the two-step austempering process.The influence of mechanics and microstructure and mechanical properties yields the following innovative results:1?Pre-forming part of martensite before isothermal can significantly shorten the incubation period of bainite transformation,and the proportion of shortening increases with the increase of pre-formed martensite voluem.The martensite is pre-formed,and a high coherence?/M phase interface is introduced to provide a favorable position for the bainite nucleation,thereby accelerating the bainite transformation rate.However,in the late stage of the bainite isothermal transformation,the pre-transformed martensite divides the austenite grains,hinders the growth of bainite,and does not accelerate the overall phase transformation of bainite.With pre-formed martensite process,the hardness of the final hardness and direct isothermal process are above 60HRC,and the impact toughness is increased by 22%and 47%,respectively.With the isothermal temperature of bainite rising to 250°C and 300°C,pre-formed martensite can accelerate the whole phase transformation process of bainite,shorten the completion time of transformation,and the higher the isothermal temperature,the more significant the acceleration effect.2?The supercooled austenite strength is the most important factor affecting the size of bainite lath.Firstly,a two-step austempering accelerated nano-bainite phase transformation process with equal supercooled austenite strength is proposed.The carbide-containing double-order isothermal process can significantly accelerate the nano-bainite transformation,shortening the bainite transformation completion time from30h to 10.5h,slightly increasing the bainite lath size,and the hardness and impact toughness are comparable to the direct isothermal process..The two-step austempering process of supercooled austenite strength without carbides shortens the bainite transformation completion time by 68%,and the hardness and strength are lower than the direct isothermal process,but the plasticity and impact toughness are both increased by more than 2 times.3?Furthermore,the influence of supercooled austenite strength,temperature and phase change driving force on the bainite slab size was established,and the bainitic ferrite size model f(T,?_y^?,?G^???),was established.Based on this model,the same bainite was proposed.A two-stage isothermal process of size.The double-step isothermal process of the strip size shortens the bainite transformation completion time by 58%,the hardness is comparable to the direct isothermal process,and the strength is increased by 200 MPa compared with the direct isothermal process,and the plasticity and toughness are also significantly improved,in accelerating the bainite transformation.At the same time has good mechanical properties.